With regard to the large number of different mobile radio frequencies that are used, it is known for at least two-band antennas to be provided for mobile radio antennas, in particular for the motor vehicle field.
A dual-band antenna for the mobile field has been disclosed, for example, in WO 99/04452. This antenna comprises two antenna elements which are in the form of rods, arranged offset with respect to one another in the axial direction, and connected to one another by means of an intermediate inductance (coil). Those ends pointing towards one another of the antenna elements which are in the form of rods, including the coil arranged between them, are fixed by means of an inductance on the one hand and by means of an externally located conductive sheath, which surrounds everything, on the other hand. This results in an LC tuned circuit between the lower and upper antenna elements. The tuned circuit can be tuned appropriately in order to ensure that, in a low frequency band, the entire antenna element device, with two antenna elements which are arranged offset with respect to one another in the axial direction, is used as an antenna while, in contrast, owing to the blocking effect of the LC tuned circuit in a higher frequency band, only the lower antenna element, with its corresponding length, acts as an antenna in the higher band. Thus, reception and transmission take place only via the one antenna element that is located at the bottom in a higher frequency band range.
However, the antenna nevertheless has a comparatively large physical height, for which reason it appears not to be very suitable, particularly for use as a physically short mobile radio antenna which can be fitted to the outside of motor vehicles. Furthermore, this antenna principle is restricted to a dual-band antenna and cannot be upgraded in the sense of a multiband antenna by means of which, for example, it is possible to receive three or four different band ranges. In general, it is therefore necessary to assess the bandwidth of this antenna as not being sufficient in many cases.
An antenna arrangement has also been disclosed, for example, in DE 201 11 229 U1. This prior publication describes an antenna arrangement for motor vehicles, which has a chassis above which a printed circuit board is arranged, to be precise in order to accommodate circuit components. One or more vertically projecting antenna elements, at least some of which are flat, are provided vertically with respect to the printed circuit board, which is aligned essentially horizontally, to be precise for reception of different services, or in different frequency bands for the mobile radio range.
The overall antenna arrangement is covered by a shroud, which may have a shape similar to fins. Antennas such as these are normally fitted to the motor vehicle bodywork metal sheets, for example at the junction between the motor vehicle roof and the rear windshield.
Furthermore, antenna arrangements are also known in which the printed circuit board that has been mentioned, together with the electronic circuitry components, filter circuits, etc., is first of all provided constructed on a more or less horizontal chassis, and the antenna elements are once again positioned in the vertical direction, at right angles to this. These antenna elements may, for example, comprise not only metallically conductive, self-supporting antenna element devices but, for example, may likewise once again be formed from a printed circuit board element, that is to say in general from a dielectric material, on which metallized surfaces are formed in order to create the antenna elements.
If the aim is now to receive in only one frequency band range, then one antenna element is sufficient. If the aim is to provide two or more services, or if, for example, the aim is to allow communication in different frequency bands in the mobile radio range, then, of course, two or more antenna elements, which are offset with respect to one another, or flat antenna elements are then provided.
The exemplary illustrative non-limiting implementation provides an antenna arrangement, in particular for motor vehicles, which, in addition to at lest one antenna device for the mobile radio range, provides at least one antenna for further services, for example a so-called DAB antenna for reception of digital broadcast radio programs, with the antenna arrangement being intended to have good reception characteristics while occupying a small amount of space overall.
The exemplary illustrative non-limiting antenna arrangement comprises, for example, an antenna with a multiband capability for the mobile radio range from 810 MHz to 960 MHz, as well as from 1710 MHz to 2170 MHz.
If a mobile radio antenna such as this were to be designed with an antenna device for reception of a further service, for example for the DAB-L band (in which case the antenna element required for this purpose would operate in a frequency range from 1452 MHz to 1467 MHz), then, in order to avoid mutual interference between the individual antenna devices for the various services and frequency ranges, attempts would be made to position these antenna devices as far away from one another as possible. This is because the mutual interference would be minimized by maximizing the horizontal distance between the antennas.
Very surprisingly, it has been found that it is possible to minimize the mutual interference between the individual antenna devices, and the lack of omnidirectionality of the polar diagrams resulting from this, not only by maximizing the separation but also by ensuring that the distance between the different antenna devices which have bene mentioned, that is to day the distance between at least one antenna device for the mobile radio range and a further antenna device for a further service, for example for reception of the DAB-L band, has at least sections which are less than lambda/8 λ (λ/8).
In one exemplary illustrative non-limiting implementation, a first antenna device, which is at least provided, and a second antenna device are arranged such that both antenna devices are arranged overall at a distance of less than λ/8 apart. In this case, “λ”, when it relates to a multiband antenna for the antenna from the mobile radio range, preferably means the wavelength in the uppermost telephone frequency band.
An exemplary illustrative non-limiting implementation provides for the antenna element terminating impedance of the mobile ratio antenna and of the DAB antenna element to be optimized such that the omnidirectionality and gain of the polar diagrams, which are subject to mutual interference caused by the two antenna devices, have optimum values. This can preferably be achieved by means of a suitable filter circuit. In the case of the DAB antenna, this terminating impedance may also be formed by a selective antenna amplifier.